1. Field of the Invention
The present invention relates to an apparatus for recording picture data on a recording medium by compressing the picture data, and more particularly, to a picture data compressing and recording apparatus for recording picture data on a recording medium by applying orthogonal transform coding to the picture data.
2. Description of the Prior Art
Digital picture data such as those produced by a digital electronic still camera are written into a memory while being subjected to any of a variety of compression coding techniques in order to reduce the amount of data and therefore, to reduce the required capacity of the memory. Two-dimensional orthogonal transform coding, in particular, is extensively used because it is capable of coding picture data by a large compression ratio and allowing a minimum of distortions ascribable to coding to occur in a picture.
Specifically, in two-dimensional orthogonal coding, picture data are divided into a predetermined number of blocks, and the picture data in the individual blocks are transformed independently of each other. The transformed picture data, i.e., transform coefficients, are compared with a certain threshold value so as to discard those coefficients which are smaller than the threshold value. The transform coefficients smaller than the threshold value are treated as (logical) ZEROs thereafter. The transform coefficients other than the discarded coefficients are divided by a predetermined quantizing step size, i.e., a normalizing coefficient, thereby, quantizing or normalizing a step size basis. This is successful in suppressing the value of the transform coefficient, i.e., the dynamic range of the amplitude.
The compressed data that have undergone orthogonal transformation as stated above are made up of a portion where the frequency component is lowest, i.e., a so-called DC component and the other portion or AC component. The two components are coded by Huffman coding or similar coding technology to become further compressed data.
Orthogonal transform coding features a large compression ratio, as discussed above. With such a coding principle, however, it is impracticable to display a picture directly by reproducing the compressed data. Specifically, decoding and orthogonal inverse transformation have to be applied to the compressed data at the time of reproduction, resulting in the need for a prohibitive amount of arithmetic operations. Hence, reproducing a picture represented by compression coded data by decoding the data is undesirable when one desires to see a picture picked up by an electronic still camera, for example, immediately as a simplified picture on a viewfinder.